To obtain hydrocarbons such as oil and gas, wellbores (also referred to as the boreholes) are drilled by rotating a drill bit attached at the end of a drilling assembly generally referred to as the “bottom hole assembly” (BHA) or the “drilling assembly.” The wellbore path of such wells is carefully planned prior to drilling such wellbores utilizing seismic maps of the earth's subsurface and well data from previously drilled wellbores in the associated oil fields. Due to the very high cost of drilling such wellbores and the need to minimize time actually spent drilling and wireline logging wells, it is essential to gain as much information as possible during drilling of the wellbores. Information about downhole conditions and materials may be acquired with wireline tools or bottom hole assemblies (BHA). Wireline tools are generally used after a wellbore is drilled, bottom hole assemblies may be used while the well is being drilled as part of the drilling string. Downhole wellbore information acquired from BHA components may be utilized, among other things, to monitor and adjust the drilling direction of the wellbores or to detect the presence of geologic formations and hydrocarbons.
In logging while drilling through an earth formation, it is desirable to measure formation shear wave velocity. The shear wave velocity of earth formations provides information important for exploration and production of oil and gas from the formation. The shear wave velocity profile enables the conversion of seismic shear wave time sections to depth sections and is utilized in the interpretation of seismic wave amplitude variation versus detector offset. The ratio between the shear wave velocity and the compressional wave velocity is closely related to the rock lithology and is related to hydrocarbon saturation. Shear wave velocity is also used to evaluate the mechanical properties of the formation in reservoir engineering applications.
Because of the importance of earth formation shear velocity, various methods have been developed to measure it. In conventional wireline logging using a monopole acoustic tool, the shear velocity can be measured from the shear wave refracted along the borehole wall if the formation shear wave velocity is greater than the borehole fluid acoustic velocity. A formation that has a shear wave velocity faster than the borehole fluid is called a ‘fast formation.’ However, in a formation where the shear velocity is slower than borehole fluid velocity, a ‘slow formation,’ the shear wave can no longer refract along the borehole wall, and the shear velocity cannot be directly measured from monopole logging. Because of the need to measure shear velocity in slow formations, especially in the soft sediments of deep-water reservoirs, dipole acoustic logging tools were developed. The dipole tool induces and measures the bending or flexural wave motion in the formation. In a sufficiently low frequency range (1-3 kHz), the flexural wave travels at the shear velocity of the formation, regardless whether the formation is fast or slow. This allows for direct measurement of formation shear velocity using the dipole acoustic tool. Dipole acoustic logging is now a mature technology with worldwide commercial applications.
An alternative technique for shear wave velocity measurement is using the quadrupole shear waves. A quadrupole acoustic tool induces and measures the quadrupole shear wave in the formation. The low-frequency portion of the wave travels at the formation shear wave velocity, allowing for direct shear velocity measurement from the quadrupole wave. Although the quadrupole shear wave has been extensively studied theoretically and a wireline quadrupole-logging tool was also proposed (in U.S. Pat. No. 5,027,331 to Winbow et al.), this technology has not yet been commercially applied to the oil and gas industry. This is largely because the wide acceptance and success of the dipole shear wave technology have fulfilled the needs for measuring shear velocity in slow formations.
The acoustic Logging-While-Drilling (LWD) technology has been developed in recent years out of the needs for saving rig-time and for real-time applications such as geosteering and pore pressure determination, among others. The LWD acoustic technology is aimed at measuring the compressional- and shear-wave velocities of an earth formation during drilling. This technology has been successful in the measurement of compressional wave velocity of earth formations. The need for determining the shear wave velocity in slow formations calls for further development of the technology for shear wave measurement capability. Because of the popularity and success of the dipole shear wave technology in wireline logging, this technology is naturally extended to the LWD situation and a LWD dipole acoustic tool has been built and offered for commercial applications.
The application of the dipole acoustic technology to LWD has a serious drawback caused by the presence of the drilling collar with BHA that occupies a large part of the borehole. The drawback is that the formation dipole shear wave traveling along the borehole is severely contaminated by the dipole wave traveling in the collar.
U.S. patent application Ser. No. 10/045,263 of Tang et al, having the same assignee as the present application and the contents of which are fully incorporated herein by reference, teaches the use of quadrupole LWD tool for determination of shear velocities of earth formations. The advantage, as taught in Tang, is that the quadrupole wave, when excited at low frequencies, travels at the formation shear velocity and is free of the tool (quadrupole) wave contamination. The LWD quadrupole waves, however, can be quite dispersive. The typical dispersion characteristic is that, as frequency increases, the quadrupole velocity departs from the value of formation shear velocity and monotonically decreases with frequency. The degree of dispersion, or the departure from the formation shear velocity, of the measured quadrupole wave velocity depends on the measurement frequency range, drilling mud used, and drill collar and borehole sizes. When there is a significant dispersion effect in the measured data, a dispersion correction procedure is needed.
Shear wave velocity measurements have also been used for determination of azimuthal anisotropy in earth formations. Such azimuthal anisotropy may be indicative of stress distributions in the earth or of fracturing in the earth. In either case, knowledge of the anisotropy is important for reservoir development. U.S. Pat. No. 4,832,148 to Becker et al. teaches the use of cross-dipole acoustic logging for determination of the direction and extent of azimuthal anisotropy. Such cross-dipole measurements are commonly referred to as 4C (for four component) data, the four components being called xx, xy, yx, and yy. The first and second letters refer to the source and receiver orientation respectively in a Cartesian system wherein the z-axis is vertical (or perpendicular to bedding).
For example, xy means emitting a dipole wave from the x-direction source and recording the wave using the y-direction dipole receiver. This indicates that the dipole measurement is a directional measurement and that is why it allows for measuring azimuthal shear velocity changes of the formation. The 4C measurement logging, is now a mature technology in wireline logging. The LWD 4C dipole measurement, however, has two important issues that must be solved. The first is that the LWD dipole, as discussed above, does not directly measure formation shear velocity because of the drill-collar interference. The second is that the rotation of the tool during drilling obscures the directionality of the dipole measurement. (Because of the drill-bit rotation, the dipole source/receiver does not point to a fixed azimuth.)
There is a need for a method of determination of shear wave velocities of earth formations that is relatively robust in the presence of tool mode waves propagating along the drill collar. The need is particularly acute in situations where the formation shear velocity is less than the velocity of propagation of compressional waves in borehole fluids. There is also a need to determine the shear-wave anisotropy azimuthal with respect to the borehole, as this information is important for detecting formation fracture system and characterizing ambient stress field. Such an invention should account for the dispersive nature of the acoustic signals in the LWD environment. Such an invention should also preferably be able to determine azimuthal anisotropy in LWD measurements. It is further desirable that such a method should account for the presence of a drill collar within the borehole, the type of drilling mud used, the size of the borehole, and eccentricity of the logging tool within the borehole. The present invention satisfies this need.